Shielding arrangement to protect a circuit from stray magnetic fields

ABSTRACT

A shielding arrangement for protecting a circuit containing magnetically sensitive materials from external stray magnetic fields. A shield of a material having a relatively high permeability is formed over the magnetically sensitive materials using thin film deposition techniques. Alternatively, a planar shield is affixed directly to a surface of semiconductor die containing an integrated circuit structure.

REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 10/314,377, filed Dec. 6, 2002, which is a divisional application ofU.S. application Ser. No. 09/668,922, filed Sep. 25, 2000, now U.S. Pat.No. 6,515,352, issued on Feb. 4, 2003, the disclosures of which arehereby incorporated by reference in their entireties herein.

BACKGROUND OF THE INVENTION

The present invention relates to shielding for semiconductor devicesand, more particularly, to shielding for semiconductor devices havingmagnetic materials used therein which are to be protected from strayexternal magnetic fields.

Magnetic materials are used, for example, in magnetic cell memories andmagnetic field sensors. In random access magnetoresistive memories,storing data is accomplished by applying magnetic fields and therebycausing a magnetic material in a cell to be magnetized into either oftwo possible memory states. Recalling data is accomplished by sensingresistance changes in the cell when magnetic fields are applied. Themagnetic fields are created by passing currents through strip lines(word lines) external to the magnetic structures, or through themagnetic structures themselves (sense lines).

Material layers which have a high magnetic permeability have been usedin monolithic integrated circuits as a basis for magnetic cell memories.Early magnetic memory cells used a magnetic permeable layer formed of athin film of a metallic alloy composition which, for example, mightinclude nickel, cobalt, and iron. The films are fabricated in the courseof the fabrication procedures for monolithic integrated circuits withsome added steps. The films so fabricated usually exhibit uniaxialanisotropy magnetoresistance, and the materials used to form such filmsare known as AMR materials. More recently, magnetic memory cells havebeen formed as narrow stripes etched into an inhomogeneous conductor,for example, a multi-layer thin film stack permalloy-copper-permalloy.Such memory cells exhibit a pronounced decrease in electrical resistancewhen an applied magnetic field brings the magnetic moments in differentregions into alignment. The materials used to form these more recentmemory cells are referred to as Giant Magnetoresistance (GMR) materials.Because very large demagnetizing fields would otherwise result, themagnetization of such thin films, whether AMR materials or GMRmaterials, will always lie substantially in the plane of the film; thatis, the magnetization vector for the material will be substantially inthe plane of the film. The orientation of the easy axis of magnetizationcan be chosen if the film is deposited in the presence of a magneticfield oriented in the selected direction.

The magnetization of thin films formed of either AMR or GMR materialswill always lie substantially in the plane of the film, that is, themagnetization vector for the material will be substantially in the planeof the film. The orientation of the easy-axis axis of magnetization canbe chosen if the film is deposited in the presence of a magnetic fieldoriented in the selected direction.

Magnetic field sensors are typically configured as a Wheatstone bridgeconfiguration. That is, all four legs of the bridge lie in a plane andchange resistance proportional to an applied magnetic field.

A shield for protection from magnetic fields may be formed of a metalhaving a relatively high permeability. One such metal which is wellknown for use in magnetic shielding, and has a high initialpermeability, is known as Mu metal and is available from CarpenterTechnology Corporation, Carpenter Steel Division. Such alloys arereferred to generally as Mu metal and are available from other sources.

U.S. Pat. No. 4,953,002 entitled “Semiconductor Device Housing withMagnetic Field Protection” dated Aug. 28, 1990 and assigned to HoneywellInc., describes a housing for integrated circuit structures containingmagnetic thin film which has permeable protective layers parallel to thethin film. U.S. Pat. No. 5,939,772 entitled “Shielded Package ForMagnetic Devices” dated Aug. 17, 1999 and assigned to Honeywell Inc.,describes the use of permeable metal shields attached by epoxy to theoutside of a high-reliability hermetic package. If the shields had beenlocated in the die cavity, exposure of the epoxies to the high assemblytemperatures could have liberated large amounts of moisture, which wouldhave resulted in early failure of the integrated circuit.

When a magnetic field shield is located outside the package, the shieldextends beyond the underlying magnetizable material by an amount that issomewhat related to the spacing of the shield from the magnetizablematerial. For example, if the distance from the plane of themagnetizable material to the plane of the shield is 0.015 inches, thenthe size of the shield may be selected so that it extends beyond themagnetizable material by two or three times this amount or 0.030 to0.045 inches. Therefore it is desirable to locate the shield as close aspossible to the magnetizable material so as to minimize the use ofshield material and the associated weight and cost.

In integrated circuit devices having such permeable thin films, theorientation of the magnetization vector in the plane is usuallyimportant to the operation of the device. In accord with thermodynamics,the magnetization in such a film will arrange itself to minimize themagnetic energy. Magnetic fields external to the film will often begenerated in and about the device as part of the device operation. Thesefields must be oriented to have components in the plane of the magneticthin films to have a significant effect on the magnetization of suchfilms in accord with minimizing the magnetic energy. Fieldsperpendicular to the films will have no effect on such magnetization.

With regard to stray magnetic fields, i.e., those magnetic fields whichare generated from sources external to the film and to the integratedcircuit device and its housing, there will be a desire in many instancesthat part or all of them have no significant effect on these permeablefilms. This is particularly true in the case of memory devices where theinformation contained in the memory is contained in the orientations ofthe magnetization vectors of the magnetic material used in each memorycell. Any such external magnetic field effects which would alter theorientations of the magnetization vectors in the memory cells couldcontribute to a loss of information or to erroneous information beingprovided by the memory. Recent improvements in magnetic film memoriesmay lead to their widespread use in commercial devices. Therefore, suchfilms need to be protected from external magnetic field disturbances,but the integrated circuit structures must also be housed in such a wayto minimize cost if they are to be a viable product for the commercialmemory market. Therefore, a shielding arrangement to protect magneticfilms in such integrated circuit structures from significant externaladverse influences, including external magnetic fields, and which can beeconomically provided, would be desirable.

For military, space or other applications requiring a high reliabilitypackage, it is desirable to have a hermetically sealed package that isfree from any internal organic materials such as epoxy materials thatmay liberate moisture. In applications such as the radiation environmentof space, it is also desirable to have the metal parts within thepackage at V_(SS) or ground potential.

Thus a need exists for a simple, lightweight, economical shieldingarrangement for integrated circuits using magnetizable materials.

SUMMARY OF THE INVENTION

The present invention solves these and other needs by providing ashielding arrangement for protecting a circuit from stray magneticfields, including a circuit die having an integrated circuit structurethat contains a magnetizable material with its magnetization orientationconfined substantially to a magnetization plane. In a first aspect, thedie has a surface parallel to the magnetization plane. Magneticshielding material located at the surface of the die is affixed to thedie and is of a size to overlay the magnetizable material. In anotheraspect, the invention includes a magnetic field shield formed by thinfilm processing techniques. Multiple layers of magnetic field shieldingmay be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top plan view of a shielding arrangement according to theteachings of the present invention.

FIG. 2 shows a cross-sectional view of the shielding arrangement of FIG.1 according to section line 2-2 of FIG. 1.

FIG. 3 shows a cross-sectional view of an alternate embodiment of thepresent invention.

FIG. 4 shows a top plan view of an alternate embodiment of the presentinvention.

FIG. 5 shows a cross-sectional view of the shielding arrangement of FIG.4 according to section line 5-5 of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A shielding arrangement for protecting a circuit die containing amagnetically sensitive circuit from stray magnetic fields is shown inthe drawings and generally designated 10. FIG. 1 shows a top plan viewof an integrated circuit die 12 including upper surface 14, sides 26,lower surface 16, power supply pads 18, and signal input/output pads 20.Die 12 is shown in a ceramic package 30 after the die attach process.Die 12 further includes an area having an underlying magneticallysensitive circuit 22. Magnetically sensitive circuit 22 includesmagnetizable material with its magnetization orientation confinedsubstantially to a magnetization plane defined by the plane in which themagnetizable material lies. Surface 14 of die 12 is parallel to themagnetization plane 22. Electrical connections 24 in the form of wirebonds extend from bonding pads 20 to a connection site on package 30.Shielding arrangement 10 may take various forms and a single-chippackaging arrangement for a magnetoresistive memory device will bedescribed first. In this shielding arrangement, shield 40 is a thinplanar shield of Mu metal, for example, about 0.004 inches thick. Shield40 includes top surface 42, bottom surface 44, and sides or edges 46. Inaccordance with the principles of the present invention, shield 40 isattached directly to surface 14 of die 12. Shield 40 is preferablyattached to surface 14 using cyanate ester, which is available in pasteform from Matthey Bishop Company and other suppliers. It is availablefor non-conductive applications or with silver loading for conductiveapplications. Attachment of shield 40 is typically done at about 150degrees centigrade. Shields thicker or thinner than about 0.004 inchesmay, of course, be used.

Shield 40 is of a relatively high permeability material. Magnetic fieldportions oriented toward the sides 26 of die 12, and so in themagnetization plane, will tend to pass through shield 40 rather thanthrough die 12 because of the high permeability of shield 40. Inaddition, there are distinct advantages to locating shield 40 as closelyas possible to the plane of the magnetic materials. In magnetoresistivememory applications, the magnetic materials are formed after theunderlying electronics are formed. The additional semiconductoroperations necessary to form and pattern the magnetic materials ofmagnetizable circuit 22 necessarily place the magnetic materials veryclose to an upper surface of die 12 during processing. Therefore shield40 is spaced very close to the plane of the magnetic materials andextends beyond the magnetic materials located below. Attaching theshield closer to surface 14 of die 12, and therefore closer tomagnetization plane 22, allows the use of a thinner, lighter, and moreeconomical shield than if a shield were located outside package 30, orif the shield were located beneath die 12 at surface 16. If a shieldwere located outside package 30, it would be spaced significantly fromsurface 14. If a shield were located beneath die 12, it would be spacedthe thickness of die 12 from surface 14. Magnetic field portions whichare oriented perpendicular to surface 14 and therefore to magnetic plane22 will pass through die 12 and will not significantly affect themagnetization of magnetic materials in magnetizable circuit 22. As aresult of the closeness of shielding arrangement 10 to the magnetizationplane, magnetic field portions oriented toward sides 26 of die 12, andin the plane of magnetic materials, will tend to pass through shield 40,and it is believed that little fringing will occur in the plane of themagnetic materials.

The use of cyanate ester as a shield bonding material allows arrangement10 to be exposed to elevated temperatures without liberating moisture,and cyanate ester will remain stable up to 350 degrees centigrade.

An additional shield 50 located at a lower surface 16 of die 12 may beused in some applications. However, depending on the application, theuse of shield 40 attached directly to silicon surface 14 may providesufficient shielding so that additional shield 50 is not needed.

When die 10 will operate in a radiation environment, for example incertain space applications, it may be desirable to ground shield 40. Thepresent invention allows shield 40 to be conveniently connected by thinwire 32, or other means, directly to ground pad 34 on die 12.

An alternate embodiment of the shielding arrangement 10 is shown in FIG.3 where reference numerals, e.g., 12 a are used for similar parts.Material for shielding arrangement 10 may be prepared by dispersing asuitable shield metal powder in an organic carrier and binder to createa shield paste 52. Shield paste 52 could then be dispensed over aportion of upper surface 14 a of die 12 a and allowed to cure. Thedispensing could occur after die attach or after wirebonding of leads todie 12 a.

As shown in FIG. 3, shield paste could be used as the die attach agent54 for die 12 a and following either die attach or wire bondingoperations, the paste could be applied over a portion of surface 14 a.

The previously described embodiments have described the presentinvention relative to operations performed at the individual die level.Shielding arrangement 10 may also be implemented at the wafer levelfollowing passivation with, for example silicon dioxide or siliconnitride. These methods include dispersing a shielding material powderinto a carrier and using a coating process to place the shieldingmaterial on the wafer. Some of the various methods of providingshielding arrangement 10 at the wafer level will be described by way ofexamples.

EXAMPLE 1

A suitable powder of shield metal in an organic carrier could beprepared into a slurry that could be silk screen printed and then cured.That is, the silk screening process would pattern the shield on thewafer to provide a shield for the individual die, while leavingelectrical connections on the die accessible. Suitable metals includeiron, cobalt, or nickel in the form of elements, alloys or oxides. Theselection of the specific metal, the size of the particles, and thedensity of distribution may be used to control the shielding propertiesincluding permeability, flux concentration, and saturation limit. Thismethod could occur after passivation of the wafer. The silk screeningprocess could be used to only coat selected areas of the wafer.

Shielding arrangement 10 may also be implemented at the wafer level byemploying semiconductor-processing techniques to form the magnetic fieldshielding material. The thin film deposition techniques may includeevaporation, sputtering, ion-beam deposition, plasma vapor deposition orepitaxial methods.

EXAMPLE 2

Magnetic field shield material may be deposited on the wafer usingphotolithographic processing. For example, magnetic field shieldmaterial may be deposited and patterned using a shadow mask of amaterial such as molybdenum or chrome.

EXAMPLE 3

Magnetic shield material may be deposited onto the wafer in a blanketdeposition. A photoresist material may then be coated onto the magneticshield material and patterned using photolithography. Magnetic shieldmaterial may then be removed using wet or dry etching techniques so thatmagnetic shield material remains where desired.

EXAMPLE 4

A photoresist layer may be coated onto a wafer. The photoresist isexposed and etched so that photoresist is removed where magnetic shieldmaterial is desired. A magnetic shield material is then blanketdeposited. The photoresist layer is then removed using a “lift-off”process to remove photoresist and magnetic shield material where themagnetic shield material is not desired.

EXAMPLE 5

A conductive seed metal layer is deposited on the wafer using atechnique such as sputtering or evaporation. A magnetic material is thenplated onto the seed layer. Photolithography is then used to pattern themagnetic shield material so that magnetic shield material remains wheredesired.

Now that some of the methods of implementing shielding arrangement 10 atthe wafer level have been set forth, many advantages can be further setforth and appreciated.

FIG. 4 shows a die 60 of semiconductor material containing an integratedcircuit structure and having a surface 61 and including bonding pads 62.Die 60 includes magnetically-sensitive circuits having magnetizablematerial. Magnetic field shield 64 functions to shield magnetizablematerial 66, and magnetic field shield 68 functions to shieldmagnetizable material 70. Shield 64 consists of a single layer ofmagnetic field shielding material. Shield 64 includes a portion 65connecting shield 64 to one of power supply ground pads 67. Shield 68includes layers 72, 74, and 76. Magnetic field shield 64 and magneticfield shield portion 72 could have been formed from a single depositionlayer, and could be formed of material that is electrically conductiveor electrically insulative. Shield portion 74 of shield 68 could be aninsulative material, and shield portion 76 could be an electricallyconductive material. Shielding arrangement 10 provides flexibility tomeet differing shielding needs. This flexibility allows shielding to beplaced only on a portion of the die having underlying magnetizablematerial if so desired. The flexibility provides for variation in thenumber of shielding layers on different portions of the die. If time andfrequency differences exist for signals in the magnetically sensitivecircuits that are to be protected, then the nature of the shielding canbe varied. This variation may result in using insulative magnetic fieldshielding material in high frequency circuits to reduce eddy currentproblems in the shielding material.

Shielding arrangement 10, whether utilizing an individual separableshield or a shield applied using semiconductor thin film processingtechniques, provides a shield that is contiguous with the integratedcircuit structure that is being protected.

Shielding arrangement 10 has been described with reference to a singlechip ceramic layer package however it is equally applicable to otherpackage types, e.g., multichip modules and low-cost plastic packages.For example, with the use of shield arrangement 10, a shield could beapplied and the die could then be sent to any commercial packaging houseto be packaged in plastic organic packages.

Thus, since the invention disclosed herein may be embodied in otherspecific forms without departing from the spirit or generalcharacteristics thereof, some of which forms have been indicated, theembodiments described herein are to be considered in all respectsillustrative and not restrictive. The scope of the invention is to beindicated by the appended claims, rather than the foregoing description,and all changes which come within the meaning and range of equivalencyof the claims are intended to be embraced therein.

1. A method of providing a magnetic shield for an integrated circuit,the method comprising: providing a die with magnetoresistive memorycells for the integrated circuit; providing a magnetic shield that isconfigured to fit within a die cavity of a package for the integratedcircuit; and bonding the magnetic shield to the die with cyanate estersuch that a surface of the magnetic shield is substantially parallel toa magnetization plane of the magnetoresistive memory cells of the die.2. The method as defined in claim 1, wherein the cyanate ester is loadedwith silver such that the loaded cyanate ester is conductive.
 3. Themethod as defined in claim 1, wherein the magnetic shield comprises Mumetal.
 4. The method as defined in claim 1, wherein the magnetic shieldcomprises Mu metal further comprising forming the Mu metal magneticshield to a thickness of about 0.004 inches.
 5. The method as defined inclaim 1, further comprising forming the magnetic shield and bonding themagnetic shield to the die such that the magnetic shield extends beyondthe magnetic materials of the magnetoresistive memory cells.
 6. Themethod as defined in claim 1, further comprising grounding the magneticshield.
 7. The method as defined in claim 1, further comprisinggrounding the magnetic shield to a ground pad via a thin wire.
 8. Themethod as defined in claim 1, wherein bonding the magnetic shieldfurther comprises bonding the magnetic shield to a surface of the diethat is proximate to the magnetic materials of magnetoresistive memorycells, further comprising bonding an additional magnetic shield t anopposite surface of the die.